Highly Efficient Civil Aviation, Now via Operations - AAR and Challenges

R K Nangia

doi:10.4271/2018-01-1925

Global civil aviation growth at 5+% yearly poses extreme environmental challenges. Advances have appeared gradually through improved aerodynamic shapes, using carbon fibres, and enhanced engines; however, as these technologies mature, direct efficiency advances require increasing effort. Often Passenger convenience is forgotten e.g. the long-range air traffic has developed on hub-spoke basis implying extra feeder flights, transit passenger inconveniences, capacity issues.

Efficiency metrics emphasize “Why, How & What”, with an understanding of the range sensitivities, operational concepts and performance goals via the important “X-factor”. For given range, current aircraft are “greener” than previous generations. Medium range aircraft s are always greener than those for short or long ranges. However, currently, the major trend is for the latter: twin-aisle A350, A380, B787, B777X (10+% payload, 40+% fuel to MTOW). Shorter range single-aisle aircraft are “feeders” or newer derivatives: A320, B737 class (20+% payload, 20+% fuel to MTOW).

New technologies could feature in future e.g. Natural Laminar flow, riblets, enhanced loads allevation, composite tailoring, morphing structures, distributed propulsion, bio-fuels etc. These may make significant improvements and lead to unconventional layouts e.g. blended wing bodies, high aspect ratio wings, oblique wings, and joined wings. Additionally, significant environmental gains can be made via operations e.g. AAR and Formation flying.

Air-to-air refuelling (AAR) has been practised and perfected by the Military for 80+ years. Tankers are sky “gas- stations”. The Military objective is for mission success rather than fuel economy. Tankers accompany and refuel short- range aircraft over longer missions. AAR can be a strong enabler for the civil aviation. Small dedicated tankers (A320 size) can operate over short radii, refuelling longer range cruisers. AAR will always retain top hierarchy over any technological advances, offering step change towards highly efficient aviation. We discuss the pros and cons of operational issues, routing and constraints, turbulence, air navigation and environmental impact.

Replacing today´s inter-continental system with AAR gives fuel and CO2 reductions of 15-30% depending on range. Additionally, 30-40% weight savings lead manufacturers focus on smaller aircraft. Major COC and DOC reductions of a similar order occur. Noise, emissions, wake effects are favourable, meeting ACARE/NASA goals. A by-product is that laminar-flow aircraft introduction can be eased.

Increasing AAR benefits occur as Point A to B system replaces the hub--spoke system. The smaller AAR-cruisers imply ground-based opportunities: smaller airports and new connections, easing the transit passenger handling and reducing travel time. For sustainable aviation growth and future urbanisation, short flights are replaced by other means. The capacity relief becomes available for long flights (only aviation is suitable). Maintaining transport capacity, less AAR enabled cruisers are needed; these operate at 20+% payload to MTOW. More likely is that the total airborne mass is lower.

Certification and Operational rules will need revision. New tankers or other types modified from civil aircraft respect most CS-25 regulations. We aim for automatic refuelling (as demonstrated by A330 tanker recently and as in US-UCAV programme). We allude to newer versatile twin-aisle cruisers with differing capacities operating world-wide ranges with AAR, blending with formation flying. All this should “spur/re-vitalise” Aviation. We propose practical demonstrations. A game changer in sight!

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Highly Efficient Civil Aviation, Now via Operations - AAR and Challenges 2018-01-1925

SAE MOBILUS